Activated sludge systems are widely used throughout the world to treat wastewater and to particularly remove nutrients such as nitrogen and phosphorus from the wastewater as well as reduce the BOD levels within the wastewater. See, for example, U.S. Pat. Nos. Re. 32,429 and 4,874,519, the disclosures of which are expressly incorporated herein by reference.
Typically, in an activated sludge system, wastewater influent is directed through a series of treatment zones and subjected to various forms of treatment, for example, anaerobic, aerobic, and/or anoxic treatment. After such treatment, the wastewater is directed to a final clarifier which separates sludge in the wastewater from a purified effluent. The purified effluent is discharged into a stream or lake, for example, while the sludge from the final clarifier is returned to the head of the activated sludge system and mixed with the influent wastewater to form what is commonly referred to as mixed liquor. Throughout the wastewater treatment process, the sludge from the final clarifier is recycled through the activated sludge system. The biomass or microorganisms associated with the recycled sludge act to effectively remove nutrients such as nitrogen and phosphorous and reduce the BOD levels within the wastewater being treated.
However, the sludge being recycled through the activated sludge system has to be continually removed or wasted from the process. Usually, the most convenient method of disposal of this sludge is by land applications, for example, as an additive or fertilizer to an agricultural field. However, any contaminants must first be removed or separated from the sludge. Therefore, in a typical activated sludge system or process, a portion of the sludge leaving the final clarifier, also referred to as waste activated sludge (WAS), is directed to a digester where the sludge is treated and cleaned by removing pathogens and volatile elements.
Various levels of treated sludge can be realized through the digestion process. For example, some wastewater treatment facilities desire a very clean sludge that is capable of being disposed on agricultural fields or in landfills. This digested sludge is often referred to as Class A sludge. There are, other classes of sludge that are not as clean or pure as a Class A sludge. In any event, there are numerous types of sludge digesters that can produce Class A sludge and other classes of sludge as well. One very popular digester used today is an autothermal thermophilic aerobic digestion system (ATAD). Here, the incoming waste activated sludge is subjected to a mechanical thickening process and then stored in a holding tank for metering to the ATAD system. The ATAD system consists of one to three serially-connected reactors, possibly in multiple series, through which the waste activated sludge undergoes an aerobic digestion process.
Depending on the method of disposal of the final product, the treated waste may be in different forms. As stored in a post-ATAD holding tank, the sludge contains a significant amount of water and is used for such applications as roadside watering. In further post-ATAD processing, the sludge may be subjected to a dewatering process to obtain a more concentrated final product. When water is removed, the sludge attains the consistency of approximately that of wet soil and is used as an additive to agricultural fields or is disposed of in solid waste landfills.
The major drawback of the disposal of the un-dewatered sludge is the cost of transporting the treated waste. Since a majority of the composition is water, the majority of the transport cost is a direct result of the water content of the sludge. Furthermore, there is a negative stigma attached to the disposal of this waste, liquid or solid. "Solid" waste has the advantage of lower transportation costs and easier means of disposal in specific isolated locations such as solid waste landfills. Thus, it is often desirable to separate as much water as possible from the sludge before disposal. Therefore, after digestion, the sludge which is often referred to as treated biosolids is directed to a dewatering facility where excess water is removed from the biosolids. The separated effluent can be routed back to the head of the plant and directed through the activated sludge system for further treatment.
Dewatering of treated biosolids is currently achieved by the use of mechanical means such as a centrifuge. A polymer is often added to the wastewater to facilitate the formation of flocs of biosolids. Since the solid component of the wastewater solution is generally anionic, the theory is that the introduction of a cationic polymer will cause bonding of the sludge with the polymer in furtherance of a charge neutralization process. Flocculation, or bonding of the sludge with the polymer, will have the secondary effect of driving water out of the resulting flocs. However, good flocculation requires thorough mixing of the polymer with the sludge. Currently, mixing of the polymer with the sludge is difficult and summarily results in a very high amount of polymer consumption to achieve even poor flocculation of the sludge. Moreover, the current methods yield high polymer costs due to the amount of polymer used and high transportation costs since the sludge is insufficiently dewatered.
One key to obtaining an enhanced flocculation effect from the polymer is to ensure thorough mixing of the polymer with the sludge. Means such as diluting the polymer with water before mixing with the sludge or subjecting the polymer/sludge mixture to significant agitation are typical examples of the necessary steps that have been taken toward increasing the efficiency of the dewatering process. However, to date, this has only resulted in limited gains in dewatering efficiency with questionable cost effectiveness.